Why Sunday's Solar Eclipse 'Ring of Fire' Will Be Rare Sight

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When the moon blots out the sun in a solar eclipse on Sunday (May
20), producing a "ring of fire" in the sky, it will cover roughly
94 percent that of the sun and offer a rare view of our nearest
star.

The remaining sun ring visible at the peak of the solar
eclipse will appear with a width of 3 percent of the
sun’s diameter. While that might not sound like a lot, it will
still be enough to drown out the beautiful solar corona, as well
as the sun’s ruby red chromosphere and any prominences — such as
plumes of hydrogen gas, leaping off of the limb of the sun.

Meanwhile, the sky (while getting somewhat dimmer) will never
darken enough to see any bright stars or planets. All of these
sights are reserved solely for a total eclipse.

Or are they?

As it turns out, we can have a wide variety of
annular solar eclipses and that ring of fire can appear quite
different depending on specific circumstances, namely the
distance of the Earth to the sun and of the moon to the
Earth.

In fact, if the Earth revolved around the sun in a perfect
circle, at its mean distance of 92.9 million miles (149.5 million
kilometers), and the moon revolved around the Earth in a perfect
circle equal to its mean distance of 238,857 miles (384,320 km),
then a total
eclipse of the sun would be impossible, because the moon as
seen from the Earth would appear too small to completely cover
the sun. So we would always see an annular or ring eclipse.

Fortunately, the moon can come closer to Earth than its mean
distance. As a result, there are times when it will be large
enough to totally eclipse the sun. In fact, there are occasions
when the moon will be at a point in its orbit where its apparent
diameter literally matches that of the sun (or very nearly so).
The result is an annular eclipse that displays some of the
characteristics of a total eclipse. [ Annular
Solar Eclipse of May 20 (A Photo Guide) ]

Skinny ring: Some of our readers may remember an
annular eclipse that swept across parts of the Southeast and
Mid-Atlantic States on May 30, 1984. On that day, the tip of the
moon’s dark umbral shadow missed the Earth by less than 500 miles
(800 km) and the moon’s disk had an apparent diameter less than
two-tenths of 1 percent that of the sun.

And because of the moon’s rugged topography, instead of a ring,
observers who were positioned along the eclipse track that day
(which included Atlanta and Greensboro, N.C.) saw an exceedingly
thin ring, punctuated here and there by bright beads of sunlight.
Those who blocked out the beads with their thumb could actually
glimpse the corona. And even though it was only a few degrees
from
the sun, Venus popped into view.

Because the ring was so delicately thin, annularity did not last
very long – less than 10 seconds.

Fat ring: In contrast, on Dec. 24, 1973, an
annular eclipse swept across parts of Mexico, Central America and
Africa. It occurred around the time that the moon was at its most
distant from Earth (called apogee), so it appeared to be very
small. But it also took place near the time that Earth was near
its closest point to the sun (called perihelion), so the sun
appeared larger than normal.

In the end, the moon appeared 9 percent smaller than the sun,
creating a very "fat" ring nearly 5 percent of the sun’s diameter
in width and causing the ring phase to last almost 12
minutes for some. With so much of the sun left uncovered, the sky
didn't get overly dim; some who witnessed it later said that if
you didn't know in advance that an eclipse was taking place, you
wouldn’t have noticed it at all. [ Quiz:
How Well Do You Know the Sun? ]

Sunday’s eclipse: A typical annular

Sunday’s eclipse falls in the middle of these two extremes.

The moon will be near its apogee point, making it appear rather
small, but the sun is also approaching its farthest point from
Earth (aphelion) in early July, making it appear somewhat smaller
than normal and thus compensating for the moon’s smaller than
normal size. So the ring will not be as wide as it was in 1973,
but it will be nowhere near as thin as it was when it was almost
total in 1984.

The hybrid solar eclipse will start as an annular over the
Atlantic Ocean about midway between Bermuda and Puerto Rico then
transition into a total eclipse as the moon’s shadow heads for
Africa. That eclipse will result in a partial eclipse at sunrise
for the U.S. East Coast. And for some places, like Boston
(which will not see any of this Sunday’s solar show), it will be
their first solar eclipse since the year 2000.

Mark your calendars!

Editor's note: If you snap any amazing solar eclipse photos
that you'd like to be considered for use in a story or gallery,
please send images and comments to SPACE.com managing editor
Tariq Malik at tmalik@space.com.

Joe Rao serves as an instructor and guest lecturer at New
York's Hayden Planetarium. He writes about astronomy for The New
York Times and other publications, and he is also an on-camera
meteorologist for News 12 Westchester, New York.